continuous integration: Remove python 3.6, add python 3.10

[tbenthompson]

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[codecov-commenter]

Codecov Report

Merging #68 (1d45ab6) into stable (0edf927) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           stable      #68      +/-   ##
==========================================
+ Coverage   84.34%   84.37%   +0.03%     
==========================================
  Files          11       11              
  Lines         479      480       +1     
==========================================
+ Hits          404      405       +1     
  Misses         75       75              

Impacted Files	Coverage Δ
cppimport/__init__.py	76.59% <0.00%> (+0.50%)	⬆️

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